Standardized testing of prostheses usually involves various tests that are designed to assess the mechanical and physical characteristics of a prosthesis. Typical tests include static proof tests, static failure tests and cyclic tests. Static proof tests assess the structural strength of the prosthesis by applying specified loads to the prosthesis according to a particular standard. Static failure tests also assess the structural strength of the prosthesis, but in this case, the maximum force applied before failure occurs is recorded. Cyclic tests are the most relevant tests for fatigue testing. In a cyclic test, the prosthesis is placed in a testing machine and testing forces are applied at a specified frequency (cyclic repetition rate) according to a particular test regimen. The desired frequency is often in the range of 1.3 Hz to 0.5 Hz (0.8 to 2 seconds period). Cyclic fatigue tests typically run for hundreds of thousands to millions of cycles.
Similar tests are also applied to natural biological joints, such as human or animal knees, hips, ankles, shoulders, and elbows, among others, for research purposes. Testing live subjects can present practical difficulties and often is morally unacceptable, so the majority of such testing is performed on joints from cadavers or animal carcasses. Such cadaveric testing is often called “in vitro” testing. Some typical testing goals are research into the kinematics, constraint behavior, and pathologies of natural joints. Cyclic testing is also useful in such studies.
The invention applies to cyclic joint testing using joint simulator machines. In the field of the invention, such machines are referred to using a variety of phrases such as “prosthesis testing machine,” “prosthetic joint simulator,” “testing machine,” “joint simulator” and the like. Such a machine is in many cases suitable for testing either biological or prosthetic joints, because often similar test protocols are applicable to both biological and prosthetic joints. The invention applies to a range of machines intended for the testing of biological joints, prosthetic joints, or both biological and prosthetic joints.
For brevity, in the remainder of this document, the term “joint” should be understood as applicable equally to a biological or prosthetic joint unless the term is specifically limited. Similarly, although specific discussions and examples may focus on a biological or a prosthetic joint in order to give concreteness, it should be understood that the application of such discussions and examples to either biological or prosthetic joints is within the scope of the invention.
The testing of a joint occurs according to a defined set of displacement and force commands. The term “force” includes both linear forces and rotational torques. Also, the term “displacement” includes both translational and rotational movements. In many cases, the commanded forces and displacements are defined in a test regimen produced by a recognized standards body, for example those prescribed by ISO or ASTM. In other cases the commanded forces and positions may arise from clinical observations, or from engineering study requirements. In almost all cases, the tested joint is modeled as a set of two or more rigid bodies with defined kinematic degrees of freedom. A typical testing machine is described in U.S. application Ser. No. 13/295,610 filed on Nov. 14, 2011 and entitled METHOD AND APPARATUS FOR JOINT MOTION SIMULATION, the contents of which are expressly incorporated herein by reference. The test waveform defines up to six axes of motion for each body and prescribes forces or displacements referenced to these axes. Performing the test includes operating the testing machine so that the forces and displacements it applies to the tested joint conform closely to those prescribed by the test waveform.
For a valid test, the actual testing forces and displacements applied to the joint should agree well with those prescribed by the test regimen. This requirement implies that the joint testing machine must be capable of frequency response commensurate with the demands of the test regimen. Fourier analysis reveals that the prescribed waveforms typically have spectral energy at frequencies considerably above the cyclic repetition frequency of the test, often at ten or higher harmonics of the repetition frequency. Intermittent contact between the parts of a joint is often encountered by prosthetic devices in service, and therefore its inclusion in the testing regimen is mandatory. Yet, intermittent contact is a particularly challenging condition to reproduce faithfully in a testing machine, because usually it is associated with near-impulsive forces having considerable high-frequency spectral content. A system that can improve the speed and accuracy with which actual test forces and displacements converge to the values prescribed by the test regimen is a useful addition to the field.